How Sway Bars/Anti-Roll Bars Work?

Normally, without a
sway bar when the car corners the weight of the chassis shifts toward
the outside of the turn compressing the springs on that side. The
springs on the inside generally extend a little, or do nothing.
Relatively to the chassis itself, it appears that the outside suspension
compresses and the inside doesn't.

A sway bar couples
the suspensions on each side to each other, *AND* relative to the
chassis. If you could put the car up on a lift and actually compressthe
suspension on one side by hand, then a sway bar makes the compression of
one side also try to compress the suspension on the other. Ok.. it's
still not really obvious why that's useful so I'll say the same thing a
different way.

A sway bar effectively increases the
spring rate on whichever side is compressed the MOST. If the sway bar
were absolutely solid with no twist so there's a 100% coupling between
each side then an attempt to compress one spring actually becomes an
attempt to compress both springs. It doubles the spring rate. If the bar
has some twist, then it may only increase the spring rate by say 50% on
whichever side is compressed the most. So you're driving down the road
and you go over a bump that goes across the entire lane. The sway bar
does nothing. Both sides compress normally. You go around a corner and
the chassis starts to lean and compress the outside suspension and now
it's as though you have a bigger spring out there, so the car remains
more level. That's the good part. Here's the bad part. You hit a bump
with only one side, and it behaves the same way, as though you have a
stiffer spring, so you feel uneven bumps more. You feel it crossing
anything diagonally as well, such as coming into or out of a parking
lot or driveway curb.

That's all the simple "How does a sway bar work?" part. The real tricky one is.. "What does a sway bar do?"
1.
We know it keeps the car more level. So what? Limiting the lean of the
body is good because it means that when you take a quick set into a
turn, that the body isn't still moving sideways after the tires at their
limits. Otherwise you turn in quickly, the tires grip, then the body
finally finishes leaning, when it stops, the tires loose grip. This is
especially noticeable in most cars in the slalom where you lean one way
then the other and so forth.

2. It limits camber
changes. The camber is the angle that the tire leans in or out at the
top relative to the chassis of the car. The camber directly impacts the
angle at which the tire cross section meets the road and thus controls
lateral grip. As the suspension compresses the camber angle generally
changes relative to the chassis. With a normal Macpherson strut that
hasn't been lowered, the camber goes from positive to more negative as
the lower A arm swings out straight, and then back to positive as it
swings up. That swing up into positive camber is BAD. At that point the
chassis is already leaned over so the tire may be starting to roll onto
its sidewall. Changing the camber even more positive
just just
nasty. A big sway bar will prevent the body roll in the first place, and
prevent the suspension compression on the outside which causes the
positive camber change relative to the chassis.

3.
Transfer lateral grip from one end of the car to the other. This one is a
real trick to understand, but racers exploit this EVERY time they go on
the track. Their spring rates are often so high, the cars so low, and
their suspension travel so little, that the whole camber and body lean
problem is already a non-issue. The car doesn't lean much with 500 lb
springs.

They use their bars to change the balance of the car. Here's the simple rules first.
A
big bar on the front, increases rear lateral and motive traction. A big
bar on the rear, increases front lateral and motive traction.The
applications. If the car is understeering, decrease front bar size, or
increase rear bar size. This increases front lateral grip and decreases
rear lateral grip giving the car a more neutral to oversteer feel.
Reverse the process for too much oversteer. I mentioned motive grip.
That's the neat one. Let's say your RWD car is handling ok, but
everytime you get into a corner hard and get on the gas the rear inside
tire breaks looseand spins. You can't accelerate out of the turn. You
can go around the turn quite quickly, but you can't accelerate out, and
the guy with traction hooks up and passes you halfway down the next
straight because he came out of the turn going 3-4mph faster.

The
reason you're losing the traction at the inside rear, is usually
because the rear bar is too big. As the rear outside suspension
compresses, it's actually causing the rear inside suspension to compress
as well (because the bar couples the sides.. remember where we
started), and that decreases the weight on the rear inside tire. First
thing. Decrease size of rear bar. That decouples the sides a bit, let's
the inside tire press down on the road more and thus not spin when
you're on the gas.

Here's where it gets really tricky.
If decreasing the size of the rear bar doesn't help enough the next
thing you do is increase the size of the front bar. When the outside
front compresses in a corner, it causes the inside front to compress and
may actually lift that tire completely off the ground. The car is now
sitting on 3 tires and guess where the weight that was on the inside
front goes? Outside front? Some of it. The rest goes to the inside rear
where we need more grip. The total weight of the car hasn't changed.
It's just been redistributed, and a sway bar at one end, actually
transfered weight to the other end of the car. Here it is in action on a
RWD car.

See
the inside front tire off the ground. That translates into more motive
grip at the rear, and thus more acceleration, and believe me, that car
rockets out of corners.

All of this trickery applies to
a FWD car too, and since the front tires share all of the motive AND
most of lateral traction (because most of the weight is in front), all
the things that happen with big bars at either end are even more
extreme. A big front bar stabilizes the body lean more but also creates a
lot more understeer, and may make the inside front tire spin madly
under power in a corner. A big rear bar can't give you back much lateral
grip up front, but it can give you back some motive traction. Basically
lettting you accelerate out of the turn, even when the front end is
sliding pretty badly.

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